All-optical logic gates using nonlinear elements claim set v

ABSTRACT

An all-optical logic gates comprises a nonlinear element such as an optical resonator configured to receive optical input signals, at least one of which is amplitude-modulated to include data. The nonlinear element is configured in relation to the carrier frequency of the optical input signals to perform a logic operation based on the resonant frequency of the nonlinear element in relation to the carrier frequency. Based on the optical input signals, the nonlinear element generates an optical output signal having a binary logic level. A combining medium can be used to combine the optical input signals for discrimination by the nonlinear element to generate the optical output signal. Various embodiments include all-optical AND, NOT, NAND, NOR, OR, XOR, and XNOR gates and memory latch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/388,813, filed on Feb. 19, 2009, which is a divisional of U.S.application Ser. No. 12/130,595, filed on May 30, 2008, now U.S. Pat.No. 7,512,302, which is a divisional of U.S. application Ser. No.11/354,468 filed Feb. 14, 2006, now U.S. Pat. No. 7,409,131. Theforegoing applications are hereby incorporated by reference in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical devices functioning as all-opticallogic gates. More specifically, digital optical signals are combined andprovided to nonlinear elements such as optical resonators or cavityswitches whose resonance frequencies are tuned to produce desired logicoutput signals.

2. Description of the Related Art

In electronic devices, logic gates composed of transistors comprise thebasic elements of digital circuits. Voltage-based inputs are received bythe gates, resulting in voltage-based output signals corresponding tothe desired logical function.

Interest has begun to emerge in recent years toward development of anoptical device that behaves analogously to electronic logic gates. Thereason for this interest is that optical signals can potentially travelfaster in integrated circuits than electrical signals because they arenot subject to capacitance which slows switching speed between logicstates. Given the ever increasing demand for faster switching, it isexpected that in the future, absent a major technological advance inelectronics, use of digital optical devices will become increasinglydesirable if not essential.

However, use of optical devices to form integrated logic circuitspresents unique challenges. By its nature light propagates and cannot bestored. The ability to represent a logic level stably for as long as maybe required becomes an issue. It would thus be desirable to provideoptical logic gates that can be used to represent logic states stablyusing optical signals.

Moreover, there is an established industry using optical componentswhich use primarily amplitude-modulated optical signals in which theamplitude or intensity of light pulses represents digital logic states.Any solution able to store and process data optically should alsoideally be compatible with existing optical telecommunicationsinfrastructure.

In some optical modulation schemes, data is represented by more than twoamplitude levels. The problem with such an approach is that it requiresvery stringent control on the amplitudes of the optical signals on whichlogic operations are performed. For example, in an AND gate, if twopulses are both at high or “1” logic levels represented by an amplitudeof “1” in this example, then the output will have an amplitude that isthe linear sum of these two levels, or “2”. A “2” is then passed on tothe logic gate of the next stage, which must be configured to accountfor a “2” representing a high logic level and a “1” or “0” representinga low logic level. Thus, the problem of two or more high levels addingbecomes more complicated and compounds as logic gates are cascaded. Itwould therefore be desirable to provide an optical circuit that avoidsthis problem.

As signals propagate through optical devices, propagation losses becomea significant problem that usually inhibits the cascading of optics.Moreover, providing gain to optical signals in a densely integratedsubstrate currently has technological and practical barriers to beingachieved. If restoration of digital optical signals could be managed inanother fashion, cascading several optical logic gates would bepossible.

Nonlinear optical cavities are typically used to perform all-opticalswitching. The term ‘nonlinear’ specifically refers to a resonatorcomprised of a material(s) whose index of refraction depends upon theintensity or power inside the resonator. The incident power depends uponthe combination of the input signals, which in turn determines the indexof refraction inside the resonator. The resonator's resonance frequencydepends upon its index of refraction as follows:

${f = \frac{qc}{2\; {nL}}},$

in which f is the resonator's resonant frequency, c is the speed oflight, L is the resonator's length, q is any positive integer, and n isthe index of refraction. The resonator's unloaded index and length canbe adjusted to a slightly different resonant frequency than the inputcarrier frequency so that only light of sufficient power can increase ordecrease the resonator's index of refraction enough to shift theresonator's resonant frequency to equal the incoming carrier frequency.Once the input light resonates within the resonator, the photons havemuch higher resonator lifetimes and a larger percentage of the input istransmitted through the resonator as an output. The ability of theresonator to readily switch from an opaque state to a transparent statebased on a designed amount of input power is why nonlinear cavities arethe most common form of all-optical switches.

Although sufficient power can switch a nonlinear resonator to transmit,even greater amounts of input power will further shift the resonator'sresonant frequency until it no longer matches the carrier frequency,switching the output off. This behavior has always been consideredundesirable, for conventional digital design requires a constant outputlevel regardless of the input level once a threshold is reached. Thecurrent thinking and state of the art in research and industry fails torecognize that this behavior could instead be used to a designer'sadvantage in a way that implementing all-optical logic would beconsidered much more favorably than it is today.

A nonlinear resonator can also function as the inverse of the detunedresonator described above by having its unloaded resonance frequencyequal the input carrier frequency. Inputs with relatively low power willthen be transmitted, while inputs of relatively high power will shiftthe resonator out of resonance and switch the output off. It has notbeen heretofore recognized that this inverting functionality of anonlinear resonator is useful if properly utilized in conjunction withother features described above.

SUMMARY OF THE INVENTION

The disclosed devices, in their various embodiments, each overcome oneor more of the above-mentioned problems, and achieve additionaladvantages as hereinafter set forth.

A logic gate in accordance with the invention receives one or moredigital, amplitude-modulated optical input signals. In some embodiments,one of the optical input signals is continuous wave (CW) light from alaser source, for example. The logic gate comprises a nonlinear elementwhich receives the optical input signals, or a combined signal resultingfrom their combination, and nonlinearly discriminates logic level togenerate an optical output signal having a binary logic level. Thenonlinear element can comprise an optical resonator or cavity configuredso that it is tuned relative to the carrier frequency of the opticalinput signals to perform a particular logic operation. In someembodiments, the logic gate comprises a combining medium to receive andcombine the optical input signals to produce the combined signal, whichthe combining medium outputs to the nonlinear element for logic leveldiscrimination. In other embodiments, the optical input signals areprovided to the nonlinear element, which effectively combines anddiscriminates their logic levels. In some of the embodiments, one ormore waveguides are used to guide the optical input signals to thecombining medium or nonlinear element. In some embodiments, one or morewaveguides can be used to receive the optical output signal from thenonlinear element and provide the same to a downstream element as theoutput of the logic gate. Logic gates can be optically coupled togetherin series to form an optical circuit capable of performing virtually anylogic function. Individual or combined logic gates are capable ofperforming AND, NOT, NAND, NOR, OR, XOR, and XNOR logic operation.

The nonlinear element acts as a switch responsive to the logic levels ofthe optical input signals and either switches its output from off to onor from on to off depending on how the nonlinear element is tuned ordetuned to the input carrier frequency. Moreover, the amount of inputpower required to switch can be altered by changing the amount ofdetuning. By properly arranging the number of inputs and customizingeach nonlinear element's unloaded and loaded resonant frequencies, aselected logical function can be implemented without using anyelectronics at competitive switching speeds. Furthermore, if continuouslight is also coupled as one of the optical input signals to a nonlinearelement, restoration of optical intensity (i.e. logic level) at eachstage of an optical circuit is possible. If the continuous light is usedto maintain the nonlinear element at maximum transmission, additionaldata pulses will shift the nonlinear element out of resonance, whichyields all-optical logical inversion. Because all-opticalintensity-restorative logic gates are possible, stable all-opticalmemory is another possible embodiment of this invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will nowbe made to the accompanying drawings, which are not necessarily drawn toscale, and wherein:

FIG. 1 is a graph of nonlinear element (e.g., an optical resonator)transmission percentage versus frequency of light input to the nonlinearelement in a case in which the intensity of the light input to thenonlinear element is insufficient to drive the nonlinear element intoresonance at its resonant frequency which is detuned relative to thefrequency of the input light.

FIG. 2 is a graph of nonlinear element transmission percentage versusfrequency of light input to the nonlinear element illustrating thenonlinear element's shift in resonance and in light transmission whenthe input light is sufficiently intense.

FIG. 3 is a plan view of an all-optical inverter (NOT gate), whichincorporates a continuous wave (CW) light as one optical input signal, adata input that is zero a second optical input signal, and a nonlinearelement (e.g., optical resonator) that is in resonant or transmissionmode. Above the nonlinear element is a graph of element transmissionversus frequency, with the vertical line representing the light'scarrier frequency.

FIG. 4 is a plan view of an all-optical inverter (NOT gate), whichincorporates a continuous wave (CW) light as one optical input signal, adata input as a second optical input signal that is on (i.e., highamplitude or logic level), and a nonlinear element in non-resonant oropaque mode. Above the nonlinear element is a graph of elementtransmission versus frequency, with the vertical line representing thelight's carrier frequency.

FIG. 5 is a plan view of an all-optical AND gate, which contains twooptical input signals with data that are at zero amplitude (i.e., lowamplitude or logic level) and a nonlinear element that is innon-resonant or opaque mode. Above the nonlinear element is a graph ofelement transmission versus frequency, with the vertical linerepresenting the light's carrier frequency.

FIG. 6 is a plan view of an all-optical AND gate, which receives twooptical input signals that are on (i.e., have data with a high amplitudeor logic level) and a nonlinear element that is in resonant ortransmission mode. Above the nonlinear element is a graph of elementtransmission versus frequency, with the vertical line representing thelight's carrier frequency.

FIG. 7 is a plan view of an all-optical NAND gate receiving opticalinput signals having data that are both one-bits (i.e., high amplitudeor logic level) and outputting an optical output signal with data havinga zero-bit (i.e., low amplitude or logic level).

FIG. 8 is a plan view of an all-optical NOR gate, which receivescontinuous wave (CW) light as one optical input signal, two additionaloptical input signals having respective data, of which either or bothare on (i.e., high amplitude or logic level), and a nonlinear element innon-resonant or opaque mode. Above the nonlinear element is a graph ofelement transmission versus frequency, with the vertical linerepresenting the light's carrier frequency.

FIG. 9 is a plan view of an all-optical OR gate with logic levelrestoration, which receives two optical input signals having data, andwhich has two inverters in series which generate an optical outputsignal with restored logic level.

FIG. 10 is a plan view of an all-optical XOR gate, which receives twooptical input signals have respective data, and comprises a nonlinearelement that is detuned half as much as in FIG. 5. Above the nonlinearelement is a graph of element transmission versus frequency, with thevertical line representing the light's carrier frequency.

FIG. 11 is a plan view of an all-optical XNOR gate, which receives twooptical input signals have respective data, and comprises a nonlinearelement that is detuned half as much as in FIG. 5, followed by aninverter as shown in FIGS. 3 and 4. Above the nonlinear elements aregraphs of elements transmission versus frequency, with the verticallines representing the light's carrier frequency.

FIG. 12 is a plan view of an all-optical NAND latch having twocontinuous wave (CW) light inputs as optical input signals, twoadditional optical input signals having respective data inputs ‘set’ and‘reset’, four nonlinear elements, and two optical output signals, Q andQ-bar.

FIG. 13 is a perspective view of an optical logic gate including aphotonic crystal supported by a bridge structure.

FIG. 14 is a detailed view of a portion of the photonic logic gate ofFIG. 13 showing the structure of the input to the photonic logic gatethat is tapered to match the modal profile of the light traveling intothe logic gate from the optical fiber core.

FIG. 15 is a plan view of the photonic logic gate of FIG. 13.

FIG. 16 is a cutaway perspective view of the photonic logic gate of FIG.13.

FIG. 17 is a plan view of an embodiment of an all-optical logic gatecomprising a combining medium for combining optical input signals, and anonlinear element implemented as a ring.

FIG. 18 is a plan view of an all-optical logic gate using ring withoutseparate combining medium.

FIG. 19 is a plan view of an all-optical logic gate implemented usingoptical fibers.

FIG. 20 is a plan view of an all-optical logic gate implemented withmirrors defining a resonator cavity with nonlinear material positionedin the cavity.

FIG. 21 is a block diagram of a generalized all-optical logic gate inaccordance with the invention.

FIG. 22 is a flowchart of a generalized method of manufacturing anoptical circuit including an optical logic gate(s) configured to receivean optical input signal(s) having a binary logic level, and having anonlinear element(s) to generate an optical output signal(s) having abinary logic level.

FIG. 23 is a flowchart of a method of operation of a logic gate usingamplitude-based nonlinear discrimination based on optical inputsignal(s) with binary logic level(s) to generate an optical outputsignal(s) having a binary logic level(s).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventions now will be described more fully hereinafter withreference to the accompanying drawings, in which some, but not allembodiments of the invention are shown. Indeed, these inventions may beembodied in many different forms and should not be construed as limitedto the embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will satisfy applicable legalrequirements. Like numbers refer to like elements throughout.

DEFINITIONS

‘Downstream’ refers to a position or element that is further along anoptical transmission path relative to a reference point. It can also beused to refer to the direction of travel of light in an optical circuitaway from a reference point.

‘Off’ or ‘low’ or ‘0’ refers to an optical signal having a relativelylow amplitude or logic level.

‘On’ or ‘high’ or ‘1’ refers to an optical signal having a highamplitude or logic level.

‘Or’ is used in its inclusive sense to mean any one, some or all of thethings preceding or following the word unless otherwise indicated bycontext. Thus, ‘A or B’ includes within its meaning ‘A’ alone, ‘B’alone, and both ‘A’ and ‘B’ together.

‘(s)’ or ‘(ies)’ means one or more of the thing meant by the wordimmediately preceding the phrase ‘(s)’. Thus, ‘signal(s)’ means ‘one ormore signals.’

‘Tuning’ generally refers to configuring a nonlinear element so that itsresonant frequency is set relative to the frequency of one or moreoptical input signals. Where mentioned specifically herein, ‘tuning’ mayalso refer to configuring the nonlinear element so that its resonantfrequency is tuned to a frequency (e.g., carrier frequency. ‘Detuning’generally refers to configuring a nonlinear element so that its resonantfrequency is set at a frequency that is different from the frequency ofan optical input signal.

An ‘optical resonator’ or ‘optical cavity’ is defined as a constructwhich traps light for a finite period of time and then either transmits,reflects, or extinguishes the light. A resonator in a photonic crystalis created by placing one or more pathways where light can exist andsurrounding those pathways with periodic structures that confine thelight to the pathways. In the case of two-dimensional photonic crystals,the periodic structures are air holes and/or semiconductor rods definedin the medium composing the photonic crystal and the pathways areusually defined by the absence of structures such as holes or rods inthe medium. A resonator may also comprise a ring-like waveguide made outof semiconductor material or optical fiber. The ring is coupled to inputand output ports. A resonator may alternatively comprise a mediumsurrounded by reflective surfaces that are either alternating dielectricmaterials, alternating materials with differing indices of refraction,surfaces that are low index terminated for total internal reflection, ormetallic surfaces. A resonator may also comprise superimposed gratingswhose reflective orders are aligned so that light can be selectivelytrapped. A resonator may also comprise a nonlinear material thatcontains an electromagnetically induced index profile which holds light(e.g., soliton or voltage induced profiles).

‘Substrate’ is a workpiece or starting material upon which a logic gateis formed. The substrate can be a wafer such as one used in thesemiconductor or microlithography industries. For example, the substratecan be composed of one or more substances including semiconductor orsemiconductor-on-insulator (SOI) substrates. Possible materials includesilicon (Si), silicon dioxide (SiO₂), gallium arsenide (GaAs), gallium(Ga), boron (B), phosphorus (P), gallium phosphide (GaP), galliumnitride (GaN), and others.

‘Upstream’ refers to a position or element that is at a position towardthe origination of light in an optical gate or circuit relative to areference point. It can also refer to a direction toward the originationof light.

‘Waveguide’ refers to any structure or combination of media that canconfine and guide light. For example, a waveguide can be an opticalfiber in which a core is surrounded by cladding with a higher index ofrefraction (RI) than that of the core, which has the effect of confininglight within a certain wavelength within the core. A waveguide can alsobe formed in a photonic crystal in which light propagates more readilyin pathways defined in the photonic crystal as opposed to areas in whichstructures are defined in the photonic crystal.

Nonlinear Element for all-Optical Logic Gate

FIG. 1 shows a graph of transmission as a percentage of the intensity ofinput light versus frequency of the input light for a nonlinear elementimplemented in this case as an optical resonator. The input light has afrequency 1 of one-hundred-ninety-two-point-nine (192.9) THz (i.e., awavelength of 1.55 micrometers). The nonlinear element has a resonantfrequency 2 of one-hundred-ninety-three (193) terahertz (THz) and isthus detuned from the frequency 1 of the input light. In FIG. 1 thepower of the input light is insufficient to cause the optical cavity toshift into resonance. Thus, the percentage of carrier frequency lighttransmitted through the cavity is relatively low, approximately zero, inthe situation represented by FIG. 1.

FIG. 2 shows a graph of transmission through the nonlinear elementimplemented as an optical resonator as a percentage of the intensity ofthe light input to the nonlinear element. In the situation representedby FIG. 2, the power of the input light is sufficiently high that thenonlinear element shifts into resonance. In other words, the opticalpower is sufficiently high and close in frequency that the lightresonates in the nonlinear element and outputs a relatively large amountof the input light, nearly one-hundred percent. As will becomesubsequently apparent, this selective resonance feature of a nonlinearelement can be used to good advantage in the logic gates subsequentlydescribed.

All-Optical Logic Gates Using Nonlinear Elements

FIG. 3 is an all-optical logic inverter (NOT gate) 10 that comprises twoseparate input media 3 and 4 which can be separate waveguides. The inputmedia 3 and 4 align or merge with a combining medium 5, which can be asingle waveguide or photonic crystal, for example. The combining medium5 is configured to channel the optical input signals A, B on respectiveinput media 3 and 4, to a nonlinear element 6 such as an opticalresonator. The optical input signal A in this embodiment is continuouswave (CW) light having constant power which is channeled into the firstoptical input medium 3, and optical input signal B isamplitude-modulated light data (e.g., a data stream) that is channeledinto the second optical input medium 4. The nonlinear element 6 isprecisely detuned so that if only the continuous wave light enters thenonlinear element, the nonlinear element shifts into resonance in whichthe element's resonant frequency 1 aligns with the frequency 2 of the CWlight and outputs light as the optical output signal on optical outputmedium 7.

In FIG. 4, an amplitude-modulated first optical input signal A (e.g., CWlight) at substantially the same frequency as the second optical inputsignal B (e.g., a pulse or digital bit) from the second input medium 4combines with the CW light in the combining medium 5 and the combinedsignal enters the nonlinear element 6. The optical power incident on thenonlinear element 6 either increases or decreases sufficiently throughconstructive or destructive interference, depending upon the phasedifference between the CW light of optical input signal A and theamplitude-modulated optical input signal B, to shift the nonlinearelement out of resonance which causes the output on waveguide 7 toswitch to an ‘off’ or low amplitude or logic level. Because the deviceof FIGS. 3 and 4 outputs the inverse of the logic level (i.e., amplitudelevel) of the received amplitude-modulated optical input data on thesecond input medium 4, it is effectively an optical logic inverter.Because the light power output of the inverter 10 is solely determinedby the power of the CW light on input medium 3 and not by thepotentially diminished data of amplitude-modulated optical input signalB on medium 4, the logic inverter of FIGS. 3 and 4 performs all-opticallogic level restoration analogously to the way in which electronicinverters or transistors tied to voltage sources can perform logic levelrestoration.

FIG. 5 is an all-optical AND gate device 20 that comprises two separateoptical input media 23 and 24 which can be waveguides aligned with ormerging into a combining medium 25 which can be a single waveguide orpathway defined in a photonic crystal, for example. The combining medium25 is configured so that it aligns with or is optically coupled tononlinear element 26. Amplitude-modulated optical input signals A, Beach modulated with respective data are channeled into respectiveoptical input media 23 and 24 and combine in combining medium 25 beforeentering the nonlinear element 26. As shown in FIG. 5, if either or bothof the optical input signals A, B has a low or ‘off’ logic level, thenthe nonlinear element 26 generates the optical output signal to have alow or ‘off; logic level. The nonlinear element 26 is sufficientlydetuned away from the carrier frequency 21 of the optical input signalsA, B so that the nonlinear element 26 output on optical output medium 27switches ‘on’ only when both inputs are ‘on’ at the same time, as shownin FIG. 6. This behavior corresponds to an all-optical AND gate. Thelight power output on medium 27 transmits twice the power of either ofits inputs, i.e., the optical output signal has a logic level that isthe addition of the logic levels of the optical input signals A, B. Thisenhanced output power could be detrimental if any subsequent devices arespecifically designed to receive the typical intensity of a logical ‘1’bit (i.e, high amplitude or logic level). Two solutions exist for thisproblem in this embodiment: either the subsequent logic gates aredesigned for twice as much power from an AND gate by further detuningany subsequent receiving nonlinear elements, or a single level follower(the AND gate of FIGS. 5 and 6 receiving CW light at one of opticalinput signals A, B and amplitude-modulated data in the other of theoptical input signals A, B, with the resonant frequency detunedsufficiently to output only half as much power when both inputs are highand only if the two signals destructively interfere, but no power ifeither is at a low logic level) or two inverters can be placed in seriesafter the AND gate in order to restore the proper output power level.

In one embodiment of the invention, an all-optical NAND gate 30 as shownin FIG. 7 results by placing an inverter such as the gate 10 of FIGS. 3and 4 following the AND gate 20 of FIGS. 5 and 6. More specifically, ifthe output medium 27 of the gate 20 of FIGS. 5 and 6 is aligned with oroptically coupled to the input medium 4 of the gate 10 of FIGS. 3 and 4,then an all-optical NAND gate 30 is produced in which the data ofoptical output signal follows NAND Boolean logic with respect to theoptical input signals A, B on input media 23, 24. Thus, if the opticalinput signals A, B both have low logic levels, then the resultingoptical output signal generated by the NAND gate 30 has a ‘high’ logiclevel, and if either or both of the optical input signals A, B has a‘low’ logic level, the resulting optical output signal generated by theNAND gate 30 has a ‘high’ logic level. More specifically, the nonlinearelement 6 is tuned with respect to the carrier frequency 1 of theoptical input signals A, B so that only if both optical input signals A,B are at a high logic level does the amplitude of the optical signal onmedia 27, 4 have sufficient amplitude upon combination with opticalinput signal C (CW light) to shift the resonance frequency 2 of thenonlinear element 6 away from the carrier frequency 1 of the opticalinput signals A, B, C so that the optical output signal on the medium 7has a low logic level. Otherwise, if either or both of the optical inputsignals A, B has a low logic level, then the optical signal on media 27,4 has insufficient power upon combination with optical input signal C toshift the nonlinear element 6 out of resonance, so that the opticaloutput signal on medium 7 has a ‘high’ logic level in this situation.

FIG. 8 is an all-optical NOR gate 50 that comprises an optical inverter10 combined with an OR gate 40. The OR gate 40 comprises two separateinput media 41 and 42 (e.g., optical waveguides or pathways through aphotonic crystal) aligning with or merging into combining medium 43(e.g., a single optical waveguide or a region of a photonic crystal). Ifthe amplitude of either or both of the optical input data signals A, Bon media 41, 42 is high (i.e., has a high amplitude or logic level),then the data on optical output signal on medium 43 is high (i.e., highamplitude or logic level). Conversely, if both the optical input datasignals on media 41, 42 are in a low logic level (i.e., low amplitude orlogic level), then the optical output data signal on medium 43 islikewise in a low logic level (i.e., low amplitude or logic level). Thecombining medium 43 is aligned with or optically connects to the inputmedium 4 of inverter 10 which can be structured and function similarlyto the device previously described with reference to FIGS. 3 and 4. Ifeither or both of the optical input signals A, B on input media 4 tothis device contain sufficient power (i.e., are in the ‘high’ or ‘one’logic level with corresponding high amplitude level), the optical outputsignal on the inverter's output medium 7 switches off (i.e., has a lowamplitude or logic level). Otherwise, the optical output signalgenerated by the inverter 10 remains ‘on’ (i.e., has a high amplitude orlogic level). Because this gate 50 terminates with an inverter 10, italso restores diminished logic levels by receiving an optical inputsignal C having constant continuous wave (CW) light on medium 3 input tothe inverter 10, which has power sufficient to restore logic levels.

In one embodiment of the invention shown in FIG. 9, an all-optical ORgate 55 comprises the OR gate 40 of FIG. 8 optically aligned or coupledto first and second inverters 10, 10′ as shown in FIGS. 3 and 4.Although the gate 40 of FIG. 8 achieves the same logical function asthis optical logic gate, the embodiment of FIG. 9 allows for logic levelrestoration, while a simple passive waveguide does not.

FIG. 10 is an all-optical XOR gate 60 that comprises two separate inputmedia 53 and 54 merging into a single combining medium 55. The combiningmedium 55 optically aligns or connects with a nonlinear resonator 56.The nonlinear resonator 56 is detuned in its resonance frequency 52 fromthe optical input data signals' carrier frequency 51 by half as much asin the case of the optical AND gate 20 of FIGS. 5 and 6 so that theoptical output signal on medium 57 switches on (i.e., has a highamplitude or logic level) only when a single optical input signal 53, 54is on (i.e., has a high amplitude or logic level). If both optical inputsignals A, B are on (i.e, have a high amplitude or logic level), thenonlinear element's resonance frequency 52 shifts too far with respectto the input carrier frequency 51 to permit transmission of light to theoptical output medium 57, and the nonlinear element 56 turns off thelight on medium 57 so that the optical output signal has a low logiclevel. If neither optical input signal A, B is on, the nonlinear element56 outputs no significant light so that the optical output signal has alow logic level. Because this gate 60 outputs light as an optical outputsignal with a high logic level if a single optical input signal A, B ison, no logic level restoration is needed to compensate for itsoperation.

In one embodiment of the invention, an all-optical XNOR gate 65 shown inFIG. 11 comprises an XOR gate 50 (FIG. 10) followed by an inverter 10(FIGS. 3 and 4). In this embodiment, if both of the optical inputssignals A, B have a high logic level, then the optical output signalgenerated by the XOR gate 50 has a low logic level. The XOR gate 50outputs the optical output signal with a low logic level on medium 57,which is input to the inverter gate 10 on medium 4 as one optical inputsignal. The other optical input signal C is CW light input to theinverter gate 10 on optical input medium 3. These signals combine inmedium 5 and the resulting combined signal has insufficient power toshift the resonance frequency 2 of the nonlinear element 6 out ofresonance relative to the carrier frequency 1 of the optical inputsignals. Accordingly, the optical output level generated by the invertergate 10 has a ‘high’ logic level. If both of the optical input signalsA, B are at a low logic level, then the optical output signal generatedby nonlinear element 56 has a ‘low’ logic level. The optical outputsignal from XOR gate 50 on optical output medium 57 is input to theinverter gate 10 on optical input medium 4, and its level isinsufficient upon combination with the optical input signal C togenerate a combined signal with amplitude sufficient to drive thenonlinear element 6 out of resonance. Accordingly, the optical outputsignal generated by the inverter gate 10 has a high logic level. Ifeither, but not both, of the optical input signals A, B has a low logiclevel, then the nonlinear element 56 is driven into resonance so thatthe optical output signal from the XOR gate 50 has a ‘high’ logic level.This ‘high’ logic level is output on medium 57, and received as one ofthe optical input signals to the inverter gate 10 via medium 4. Thisoptical input signal combines in medium 5 with the optical input signalC (CW light) and the amplitude of the resulting combined signal issufficient to drive the resonant frequency 2 of the nonlinear element 6away from the carrier frequency 1 of the combined signal (which is thesame as the optical input signals), causing the optical output signalgenerated by the XNOR logic gate 65 to switch to a ‘low’ logic level.The gate 65 thus performs XNOR logic operation on the optical inputsignals to generate its optical output signal.

FIG. 12 is an all-optical memory latch 70 comprising two optical inputsignals R and S which are optically coupled to separate NAND gates 30,30′ (FIG. 7). The optical output signals Q, Q-bar of both NANDs 30 and30′ are then connected to respective second input media 24, 24′ as thesecond inputs of the opposite NAND 30, 30′. The media 24, 24′ cross atintersection 73 in this embodiment which results in no cross-talk due toan optical cross-talk filter 74. This device operates just as a basicelectronic NAND latch does, where the following logic table isimplemented:

S R Q Q-bar 0 0 invalid invalid 0 1 1 0 1 0 0 1 1 1 Q Q-bar

Thus, logic controlled all-optical memory is possible, because theoptical output signal on output medium 7 can be set to the desired valueand then stored based on the optical input signals S and R. Because thisdevice's optical output signal is effectively generated by inverters 10,10′ using a constant continuous wave (CW) light, its logic level iscontinuously restored on every switching cycle. Therefore, the storedsignal can be recycled indefinitely, which allows for all-optical memoryperformance that already exists in electronic memory gates. Theall-optical latch disclosed in this embodiment is not the only manner ofcreating memory from logic gates and should not be considered anexclusive example of how this invention can function as all-opticalmemory.

To specifically describe the operation of the latch 70, if the opticalinput signals S, R both have low logic levels, then the nonlinearelements 26, 26′ do not receive optical input signals with sufficientpower to drive the nonlinear elements 6, 6′ out of resonance so theoptical output signals Q, Q-bar both have high logic levels due to inputof respective optical input signals of CW light on media 3, 3′. Bothoptical output signals Q, Q-bar having the same logic level isconsidered an invalid logic level of the latch 70. If the optical inputsignals S, R have low and high logic levels, respectively, then the highlogic level of the optical input signal R forces the optical outputsignal Q-bar of the NAND gate 30′ to a low logic level which is fed backto the NAND gate 30, forcing the nonlinear element 6 to resonate withthe CW light input thereto, resulting in the nonlinear element 6generating a high logic level for the optical output signal Q. If theoptical input signals S, R have high and low logic levels, respectively,then the high level of the optical input signal S forces the opticaloutput signal Q generated by NAND gate 30 to a low logic level. The factthat the optical output signal Q is fed back as an optical input signalto the NAND latch 30′ ensures that the optical output signal Q has ahigh logic level. Finally, if both the optical input signals R, S havehigh logic levels, then neither of the NAND gates 30, 30′ switches logiclevel and the logic level of the optical output signals Q, Q-bar doesnot change.

Exemplary Method of Manufacturing an all-Optical Logic Gate

Having described the structure and function of the all-optical logicgates and circuits in accordance with the invention, an exemplary methodof manufacturing an all-optical logic gate will now be described.

In FIG. 13 a silicon-on-insulator wafer substrate 80 is treated withnitrogen gas to remove dust and debris. The substrate 80 is polishedusing a grinding machine and powder. Using a molecular beam epitaxy(MBE) tool, a two-hundred to four-hundred nanometer (200-400 nm) thicklayer of silicon 81 with a surface roughness less than five nanometers(5 nm) is grown on the silicon substrate 80. The wafer 80 is then placedin an electron-beam lithography chamber. In order to prevent proximityeffects, each minimum feature area is sequentially exposed to electronetching. Each such “pixel” is selectively exposed manually without theuse of external software. The device's features are etched to defineholes 82 (only a few of which are specifically labeled in FIG. 13 thatpenetrate vertically into the substrate with two-hundred-thirty-onenanometers (231 nm) diameters and are spaced four-hundred-twentynanometers (420 nm) apart in the same row, and the adjacent rows' holesare defined in layer 81 so that they are shifted left or right bytwo-hundred-forty nanometers (240 nm), relative to the first row e.g.,to form a triangular photonic crystal lattice. Holes are not etched onlywhere light is intended to propagate within the device 10 to be formedin layer 81. The photonic crystal structure is advantageous for use informing a logic gate for several reasons. First, sub-micron pathways canbe defined in photonic crystals to quickly change the direction of lightpropagation to guide the optical signals through the pathways definingthe logic circuit. Also, photonic crystal cavities can have very highQ-factors in sub-micron to micron scales, resulting in lower powerrequirements to perform logic switching.

The nonlinear element 6 is formed by inserting holes on either side ofwhere the light is to be trapped. The nonlinear element may be tuned ordetuned from the input carrier frequency either by varying the diameterof the holes, increasing or decreasing the distance between the holes oneach side, by increasing or decreasing the number of holes on one orboth sides, or a combination of these techniques. Typically, the numberof holes and the distance between both sides will remain fixed due todesired switching powers, lifetimes, and bandwidths. Therefore, in thisspecific procedure, the diameters of the interior- and exterior-mostholes are solely modified in order to tune the resonator to transmit ata desired wavelength(s).

At the left and right edges where external light enters or exits thedevice, the substrate is etched deeply in order to gain side access tothe insulator below the device. After the entire area is exposed to formoptical logic gate(s), the wafer is immersed in hydrofluoric (HF) aciduntil the insulator directly below the device is washed away, resultingin an air-suspended membrane bridge 83 as shown in FIGS. 13 and 15.After cleaning the wafer 80, the edges are cleaved to create an opticalchip 84.

Fiber waveguides 3 a, 4 a are cleaved and polished, and index-matchingadhesive 3 b, 4 b is applied to the end of the fibers to attach same toprotrusions 3 c, 4 c. The fiber may be secured to a bridge 85 within thechip 84 by adhesive or other mechanical attachment 86 so that it is heldin place relative to the waveguide end of the optical logic gate device10. Opposite ends of the fiber can be attached at the input side to a CWlaser source or upstream signal source as appropriate to provide thelogic gate 10 with optical input signals A, B. The optical input signalsA, B are then provided to the device 10 via respective fibers 3 a, 4 a,adhesive 3 b, 4 b, and protrusions 3 c, 4 c. The optical input signalsA, B further travel through respective regions 3 d, 4 d to combiningmedium 5. Light resulting from the combination of the optical inputsignals travels to the resonator 6 where it is trapped. The resonator 6outputs the optical output signal to a region 7 d where it propagates toprotrusion 7 c into adhesive 7 b and ultimately to optical fiber 7 awhere it travels to the output end of the fiber 7 a which may terminateas the input to a subsequent logic gate or as the ultimate output of thechip 84. The output end of fiber 7 a can be spliced to another opticalfiber or optical circuit (not shown) in a similar manner as describedabove, or using a large number of techniques and devices known to thoseof ordinary skill in the art.

As shown in FIG. 14, the optical input and output media 3, 4, 7 can becoupled to respective fibers using a butt-coupling technique. In thiscase, the logic gate 10 is defined so that the defines holes graduallytaper 3 e, 4 e to be more narrow along the direction of propagation ofthe optical input signals on the input side of the logic gate 10 tomatch the modal profile of the optical output signal. At the outputmedium 7, the configuration is reversed and the taper 7 e defined byholes in the substrate 81 gradually increases or becomes wider along thedirection of propagation of the optical output signal from the device 10so that the modal profile of the output of the gate 10 matches with thatwithin the output medium 7 a. Waveguides coupling light in and out of anexternal source are slowly tapered to match the modal profile of abutt-coupled fiber.

FIGS. 15 and 16 show the optical resonator 6 of the logic gate 10 infurther detail. It is evident that the outermost holes at each end ofthe resonator 6 are smaller than the innermost hole. If other holes wereto be included in the resonator 6, then they would be positioned betweenthe outermost and innermost holes and would have the same size as theholes used elsewhere in the photonic crystal outside of the resonator 6.

Another embodiment of the invention is the logic gate device 90 of FIG.17. This logic gate 90 uses a semiconductor “wire” instead of photoniccrystals formed on an semiconductor-on-insulator (SOI) substrate 90.Photonic wires 93 c, 94 c are etched using the same equipment as above,which then directly or evanescently connect to a ring or loop wirewaveguide that act as nonlinear element 96 which in this case is acirculator or resonator. The nonlinear element 96 is tuned or detuned bydefining the circumference of the ring of the nonlinear element 96. Allother procedures are followed as in the previous example except that nomembrane bridge is built, and the device 90 remains on the insulatorsubstrate 90. In operation, optical input signals A, B travel onrespective input media 93, 94 to the combining medium 95. Morespecifically, the optical waveguides 93 a, 94 a are aligned with andoptically coupled to the wires 93 c, 94 c using adhesive 93 b, 94 b.Optical input signals A, B on respective waveguides 93 a, 94 a thustravel through adhesive 93 b, 94 b into optical wires 93 c, 94 c whichmerge into medium 95 where the optical input signals combine. Thecombining medium 95 evanescently couples the optical input signals tothe nonlinear element 96. Depending upon the logic levels of the opticalinput signals on media 93, 94, the nonlinear element 96 outputs theoptical output signal by evanescent coupling to the optical wire 97 c,through adhesive 97 b, and into the output optical waveguide 97 a, whichform the output medium 97.

The embodiment of the logic gate 100 of FIG. 18 is similar in mostrespects to that of FIG. 17 with the exception that the combining medium95 is dispensed with in FIG. 18 by optically coupling the optical inputsignals A, B through respective media 93, 94 directly to the nonlinearelement 96 where such signals both combine and interfere with oneanother to generate the optical output signal on output medium 97.

Another embodiment of a logic gate 110 of the invention shown in FIG. 19comprises optical fibers 103 a, 104 a, 106 a, 107 a. Typical single-modeoptical fibers 103 a, 104 a are used as waveguides, which then connectto a fiber 106 a comprising nonlinear material. Nonlinear fibers areusually made of weak Kerr materials such as silica. Bragg gratings 106b, 106 c are then created in the nonlinear fiber at spaced positions byexposing periodic sections of the fiber 106 a to intense light (e.g.,from a CO2 laser). The resulting nonlinear element 106 can be tuned byeither altering the length and placement of the intense light exposureor by bending the fiber to change the resonator geometry.

FIG. 20 shows an embodiment of an optical logic gate 110 which hasnonlinear element 116 comprised of nonlinear material 116 a and mirrors116 b, 116 c. The optical logic gate 110 also comprises media 113, 114,in this case illustrated as optical fibers, for transmitting opticalinput signals to the nonlinear material 116, and output medium 117 foroutputting the optical output signal generated by the nonlinear material116. More specifically, optical input signals from the media 113, 114travel to and pass through one-way mirror 116 b where they combine inmedium 115 to produce a combined signal which enters the nonlinearmaterial 116 a. The medium 115 can be air or ambient environment outsideof or within the nonlinear element 116, or it can be the nonlinearmaterial 116 a either alone or in combination with the ambientenvironment. Depending upon the resonant frequency of the nonlinearelement 116 in relation to the frequency of the optical input signals,the combined signal is either extinguished or transmitted by thenonlinear material 116 a. The nonlinear material 116 a can be composedof ester-based dyes, for example. Any combined signal passing throughthe nonlinear material 116 a propagates to and through the one-waymirror 116 c and onward to the optical medium 117 from which it isoutput from the optical logic gate 110. The cavities may be tuned byeither changing the distance between the mirrors or by changing theresonator's geometry.

FIG. 21 is a block diagram of a generalized all-optical logic gate 200in accordance with the invention. In FIG. 21 the all-optical logic gate200 comprises a nonlinear element 206 comprising one or more of anoptical resonator or optical cavity formed by photonic crystal, Bragggratings in an optical fiber of nonlinear material, a circulator, adistributed feedback (DFB) laser, or other nonlinear device. The opticalinput signals A, . . . , B (the ellipsis ‘ . . . ’ represents the factthat there may be more than two signals), at least one of which isamplitude-modulated are provided directly to the nonlinear element 206which is configured to combine the optical input signals A, . . . , Band discriminate the logic level(s) of the resulting combined signal,and generate a binary logic level at its output in accordance with thelogic operation the nonlinear element 206 is configured to perform,e.g., by setting its resonant frequency in relation to the carrierfrequency of the optical input signals A, . . . , B. Alternatively, asshown in phantom line in FIG. 21, the optical input signals A, . . . , Bare provided to combining medium 205 such as a waveguide or a pathwaywhere optical input signals A, . . . , B combine. The resulting combinedsignal passes to the nonlinear element 206 which discriminates the logiclevel in accordance with the logic operation it has been configured toperform, and outputs the amplitude-modulated optical output signal witha logic level based on the logic levels of the optical input signals A,. . . , B. Optical input media 203 can be used to provide optical inputsignals A, . . . , B from a source or upstream logic gate to thenonlinear element 206, or to the combining medium 205 and from there tothe nonlinear element 206. Optical output medium 207 can be used tooutput the optical output signal to the next logic gate in an opticalcircuit or to another downstream element.

In FIG. 22 a method of manufacturing an optical logic circuit includingone or more optical logic gates begins in Step 220 in which a logicoperation to be performed is selected. In Step 222 the optical circuitis designed with one or more logic gates and optical connections asrequired to perform the selected logic operation. In Step 224 thedesigned optical circuit is manufactured by forming the logic gate(s) ofthe circuit so that its resonant frequency(ies) are tuned to performthat part of the logic operation the logic gate is intended to performaccording to the design of the optical circuit.

As an example of the method of FIG. 22, assume the optical output signalis to be generated according to the following logic operation selectedin Step 220:

Optical output signal=(optical input signal A*optical input signalB)+optical input signal C

In Step 222 the optical circuit is designed. One design capable ofachieving the selected logic operation uses an AND logic gate todiscriminate optical input signals A, B, and the resulting output signalis input to an OR gate along with the optical input signal C to producethe optical output signal for the selected logic operation. In Step 224the AND and OR logic gates are then manufactured so that the resonantfrequency(ies) of the nonlinear element(s) are tuned or detunedappropriately from the frequencies of the optical input signals in orderto generate the desired AND and OR logic gates. The resulting opticalcircuit is then coupled so that its inputs receive respective opticalinput signals A, B, C from upstream elements, and the output is coupledto provide the optical output signal to a downstream element.

In FIG. 23 a method of operation of a logic gate begins in Step 230 inwhich optical input signal(s) are received from an upstream element suchas a laser source for CW light, an optical amplitude modulator, or anupstream optical circuit element such as a logic gate. In Step 232optical input signals are guided. Steps 230 and 232 can be performed byoptical input media for respective optical input signals. In Step 234the optical input signals are combined to generate a combined signal.This step can be performed by the combining medium. Steps 232 and 234are optional steps, as represented by phantom line in FIG. 23. In step236 the logic level resulting from the combination of the optical inputsignal(s) is discriminated to generate an optical output signal with abinary logic level having a low logic level represented by a lowamplitude or a high level represented by a high amplitude. Step 236 canbe performed by a nonlinear element of a logic gate. Finally, in Step238 the optical output signal(s) is transmitted to a downstream elementsuch as the next gate(s) in the optical circuit or another opticaldevice.

Correspondence for Means

In the appended claims ‘nonlinear element means for nonlinearlydiscriminating logic levels of optical input signals to generate anoptical output signal having binary logic levels’ refers to any ofnonlinear elements 6, 6′, 26, 26′, 56, 96, 106, 116, 206 as describedherein or equivalents thereof.

Alternatives

While “tuning” an optical resonator typically refers to offsetting theresonator's resonant frequency, this invention also considers “tuning”to also refer to other methods of changing the resonator's transmissioncharacteristics as possible means of achieving desired functionality.For example, the bandwidth, profile, or center of a resonator'stransmission might be altered by changing the quality factor or byadding additional resonance peaks into a resonator, or by changing itsgeometry or index or refraction through the application of stress,electromagnetic or piezoelectric fields, injection of charge carrierssuch as holes or electrons, injection of light or other techniques.

Although described with reference to optical signals at 1.55 microns(um) which is currently standard in the optical communications industry,it should of course be understood that the principles of the inventioncan be applied to obtain advantageous results using other wavelengths orfrequencies for the optical signals. The optical signals used with thedisclosed gates and latches need not necessarily have the samefrequency.

Although the embodiments disclosed herein are described in the contextof ‘positive logic’ in which an optical signal with a relatively highamplitude is considered a high logic level and an optical signal with arelatively low amplitude is considered to be at a low logic level,‘negative logic’ could instead be employed in which an optical signalwith relatively high amplitude is considered a low logic level and anoptical signal with relatively low amplitude is considered to be at ahigh logic level.

Although the structures described herein are two-dimensional structures,it is possible to implement the all-optical logic gates herein withsimilar functions as previously described using one- or threedimensional structures, as will be readily apparent to those of ordinaryskill in the art with the benefit of the teachings provided herein.

Although the structures defining a photonic crystal herein have beendescribed as circular holes in a medium, it should be understood thatthe reverse could be done instead of making holes in a medium, such asby making posts, columns, cylinders, cubes, spheres, or other structureson a substrate to define a photonic crystal. Furthermore, it is possibleto form a photonic crystal through selective deposition of material on asubstrate as opposed to selective etching, or a combination of thesetechniques could be used to form the photonic crystal.

Other possible configurations and functionality are disclosed incommonly-assigned US 2005/0259999 filed May 21, 2004, naming John LutherCovey as sole inventor, which is incorporated herein by reference as ifset forth in full in this document.

Many modifications and other embodiments of the inventions set forthherein will come to mind to one skilled in the art to which theseinventions pertain having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it is tobe understood that the inventions are not to be limited to the specificembodiments disclosed and that modifications and other embodiments areintended to be included within the scope of the appended claims.Although specific terms are employed herein, they are used in a genericand descriptive sense only and not for purposes of limitation.

1. An all-optical XOR gate comprising: first and second optical inputmedia for receiving respective first and second amplitude-modulatedoptical input signals, the first and second amplitude-modulated opticalinput signals having respective data with binary logic levels; acombining medium optically coupled to the first and second optical inputmedia, the combining medium configured to receive and combine the firstand second optical input signals to produce a combined signal; anonlinear element configured to receive the combined signal, thenonlinear element having an optical resonator with a resonant frequencytuned relative to the frequency of at least one of the first and secondoptical input signals so that the nonlinear element outputs light as anoptical output signal with a high logic level only if the combinedsignal results from only one of the first and second optical inputsignals having a high logic level, and the nonlinear element outputssubstantially no light as the optical output signal with a low logiclevel if the combined signal results from both of the optical inputsignals having high or low logic levels, the optical output signal thushaving a binary logic level that is amplitude-modulated; and an opticaloutput medium optically coupled to receive and output the optical outputsignal from the nonlinear element, the first and second optical inputmedia, combining medium, nonlinear element and optical output mediumformed in a photonic crystal, the first and second optical input media,the combining medium, and the optical output medium defined by theabsence of structures in the photonic crystal, the nonlinear elementoptically positioned between an output side of the combining medium andan input side of the optical output medium.
 2. An all-optical XOR gateas claimed in claim 1 wherein an output side of the combining medium andan input side of the optical output medium are optically aligned withone another.
 3. An all-optical XOR gate as claimed in claim 1 whereinthe nonlinear element is defined by first and second sets of spacedstructures bounding a region with an absence of structures to define acavity.
 4. An all-optical XOR gate as claimed in claim 3 wherein thefirst and second sets of spaced structures each include a plurality ofstructures of varying size.
 5. An all-optical XOR gate as claimed inclaim 1 wherein the photonic crystal comprises silicon.
 6. Anall-optical XOR gate as claimed in claim 1 wherein the photonic crystalcomprises gallium arsenide (GaAs).
 7. An all-optical XOR gate as claimedin claim 1 wherein the photonic crystal comprises a silicon-on-insulator(SOI) structure.
 8. An all-optical XOR gate as claimed in claim 1wherein the photonic crystal is an optical chip.
 9. An all-optical XORgate as claimed in claim 8 wherein the optical chip is mounted to abridge.
 10. An all-optical XOR gate as claimed in claim 1 furthercomprising: a first optical fiber optically coupled to an input side ofthe first optical input medium to guide the first optical input signalto the first optical input medium.
 11. An all-optical XOR gate asclaimed in claim 10 further comprising: a second optical fiber opticallycoupled to an input side of the second optical input medium to guide thesecond optical input signal to the second optical input medium.
 12. Anall-optical XOR gate as claimed in claim 10 wherein the structures ofthe photonic crystal taper at the input side of the first optical inputmedium.
 13. An all-optical XNOR gate comprising: first and secondoptical input media for receiving respective first and secondamplitude-modulated optical input signals, the first and secondamplitude-modulated optical input signals having respective data withbinary logic levels; a first combining medium optically coupled to thefirst and second optical input media, the first combining mediumconfigured to receive and combine the first and second optical inputsignals to produce a first combined signal; a nonlinear elementconfigured to receive the combined signal, the nonlinear element havingan optical resonator with a resonant frequency tuned relative to thefrequency of at least one of the first and second optical input signalsso that the nonlinear element outputs light as an optical output signalwith a high logic level only if the combined signal results from onlyone of the first and second optical input signals having a high logiclevel, and the nonlinear element outputs substantially no light as theoptical output signal with a low logic level if the combined signalresults from both of the optical input signals having high or low logiclevels, the optical output signal thus having a binary logic level thatis amplitude-modulated; a first optical output medium optically coupledto receive and output the first optical output signal from the firstnonlinear element; third and fourth optical input media for receiving aconstant continuous wave (CW) light as a third optical input signal andoptically coupled to receive the first optical output signal as a fourthoptical input signal, respectively; a second combining medium opticallycoupled to the third and fourth media, the second combining mediumconfigured to receive and combine the third and fourth optical inputsignals to produce a second combined signal; a second nonlinear elementconfigured to receive the second combined signal, the second nonlinearelement having an optical resonator with a resonant frequency tunedrelative to the frequency of at least one of the third and fourthoptical input signals so that the second nonlinear element outputs lightas a second optical output signal with a high logic level having theamplitude of the CW light if the combined signal results from the fourthoptical input signal having a low logic level, and the second nonlinearelement outputs substantially no light as the second optical outputsignal if the combined signal results from the fourth optical inputsignal having a high logic level, the second optical output signal thushaving a binary logic level that is amplitude-modulated with theamplitude of the high logic level determined substantially by theamplitude of the CW light; and a second optical output medium opticallycoupled to receive and output the second optical output signal from thesecond nonlinear element as the output of the XNOR gate, the first,second, third and fourth optical input media, first and second combiningmedia, first and second nonlinear elements and first and second opticaloutput media formed in a photonic crystal, the first, second, third andfourth optical input media, first and second combining media, and thefirst and second optical output media defined by the absence ofstructures in the photonic crystal, the first nonlinear elementoptically positioned between an output side of the first combiningmedium and an input side of the first optical output medium, and thesecond nonlinear element optically positioned between an output side ofthe second combining medium and the second optical output medium.
 14. Anall-optical XNOR gate as claimed in claim 13 wherein an output side ofthe first combining medium and an input side of the first optical outputmedium are optically aligned with one another.
 15. An all-optical XNORgate as claimed in claim 13 wherein an output side of the secondcombining medium and an input side of the second optical output mediumare optically aligned with one another.
 16. An all-optical XNOR gate asclaimed in claim 13 wherein the first nonlinear element is defined byfirst and second sets of spaced structures bounding a region with anabsence of structures to define a cavity.
 17. An all-optical XNOR gateas claimed in claim 16 wherein the first and second sets of spacedstructures each include a plurality of structures of varying size. 18.An all-optical XNOR gate as claimed in claim 13 wherein the secondnonlinear element is defined by first and second sets of spacedstructures bounding a region with an absence of structures to define acavity.
 19. An all-optical XNOR gate as claimed in claim 18 wherein thefirst and second sets of spaced structures each include a plurality ofstructures of varying size.
 20. An all-optical XNOR gate as claimed inclaim 13 wherein the photonic crystal comprises silicon. 21-27.(canceled)